Silicon photonics uses silicon as an optical medium and has been an active development area in recent years because of its potential monolithic integration with complementary-metal-oxide-semiconductor (CMOS) microelectronic circuits. Silicon is transparent to infrared light with wavelengths above about 1.1 μm and also has a very high refractive index of, for example, about 3.5. The tight optical confinement provided by this high refractive index allows for optical waveguides.
For silicon photonic components, e.g., waveguides and the like, to remain optically independent from the bulk silicon of the semiconductor wafer on which they are fabricated, it is necessary to have a layer of intervening material. Typically silica is used as an intervening material because of its much lower refractive index, about 1.44 in the wavelength region of interest, than silicon and thus, light at the silicon-silica interface will undergo total internal reflection and remain in the silicon. This construction is known as silicon-on-insulator (SOI) and the waveguides formed from this construction are commonly referred to as SOI waveguides. As such, silicon photonic devices can be made using existing semiconductor fabrication techniques, and because silicon is used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip.
An avalanche photodetector or photodiode (APD) is an excellent choice for many optical applications (e.g., optical communication, optical sensing, and optical imaging) because of its internal gain and improved sensitivity. An APD is a highly sensitive semiconductor electronic device that exploits the photoelectric effect to convert light to electricity. An APD can be thought of as a photodetector that provides a built-in first stage of gain through avalanche multiplication. From a functional standpoint, avalanche multiplication can be regarded as a semiconductor analog to photomultipliers. By applying a high reverse bias voltage, e.g., typically about 100 to about 200 V in silicon (avalanche phenomenon as such could be started at as low a voltage as 10V depending upon the diode design), APDs show an internal current gain effect of up to about 100 due to impact ionization (avalanche effect).
However, silicon is not suitable for photodetectors for optical applications using a wavelength ranging from 1.3 to 1.6 μm due to a lack of light absorption of silicon in the near infrared range. One solution is combining a germanium (Ge) layer on top of a silicon (Si) layer because of the high optical absorption of Ge and its compatibility with silicon processes (e.g., CMOS processes). Therefore, a Ge-on-Si construction is very promising for developing separate-absorption-charge-multiplication (SACM) APDs. Integrating Ge APDs onto SOI waveguides offers the advantage of low junction capacitance, efficient power transferring from the waveguide to the Ge APD, and ease of process integration. Unfortunately, current Ge APDs integrated on SOI waveguides typically result in large topographical variation, process complexity, high loss, and reduced optical sensitivity.
Accordingly, it is desirable to provide semiconductor devices including avalanche photodetector diodes integrated on waveguides with reduced topographical variation and methods for fabricating such semiconductor devices. Moreover, it is desirable to provide semiconductor devices including avalanche photodetector diodes integrated on waveguides with reduced process complexity, lower loss, and improved optical sensitivity and multiplication gain. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and this background.